Water absorbing resins have found wide use in sanitary goods, hygenic goods, water retaining agents, dehydrating agents, sludge coagulants, thickening agents and condensation preventing agents and are also useful as release control agents for various chemicals.
Water absorbing resins heretofore known include hydrolysis products of starch-acrylonitrile graft polymer, carboxymethylcellulose, cross-linked polyacrylate products and other resins such as polyvinyl alcohol, polyethylene oxide and polyacrylonitrile resins. Of these water absorbing resins, the hydrolysis products of starch and acrylonitrile graft polymers have comparatively high ability to absorb water but require a cumbersome process for production and have the drawback of low heat resistance and decaying or decomposing easily due to the presence of starch which is a natural polymeric material. Carboxymethylcellulose, polyethylene oxide and polyacrylonitrile resins still remain to be improved in water absorbing ability. Among the conventional water absorbing resins, polyacrylate cross-linked products appear promising in respect of both water absorbing ability and stability of quality.
As one of the processes for polymerizing polyacrylic acid or acrylates, an aqueous solution polymerization process is known. The polymer obtained by this process is soluble in water and must therefore be cross-linked by some method if it is desired to modify the polymer into a useful water absorbing resin. However, even if the modification is effected with use of a cross-linking agent concurrently with or after the polymerization by the above process, the resulting reaction product is in the form of a highly viscous aqueous solution or a gel containing absorbed water which is difficult to handle. Thus the product must be dehydrated (dried) for solidification to obtain a water absorbing resin in the desired form (powder). It is nevertheless difficult to dry the reaction product efficiently by the usual rotary drum roller method or spray drying method because excessive cross-linking frequently takes place due to overheating during drying or reduced cross-linking density conversely results from insufficient drying. Extreme difficulties are therefore encountered in preparing a product of desired water content and water absorbing ability stably.